Slippage detection system and method for continuously variable transmissions

ABSTRACT

A slippage detection system for a continuously variable transmission capable of continuously changing a gear ratio between an input rotation speed of an input member and an output rotation speed of an output member is provided. The slippage detection system calculate a sum of differences between an actual gear ratio calculated from measurement values of the input rotation speed and the output rotation speed and a target gear ratio over a predetermined period of time, and determines slippage in the continuously variable transmission based on the sum of differences.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of application Ser. No.10/491,042, filed on Mar. 24, 2006, which is a National Stageapplication of PCT/IB02/04001, filed Sep. 30, 2002 and claims benefit ofpriority from JP 2001-302181, filed Sep. 28, 2001, the entire contentsof each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a slippage detection system and method for usein a continuously variable transmission capable of continuously changingthe gear ratio that is a ratio between an input rotation speed and anoutput rotation speed of the CVT.

2. Description of Related Art

As a continuously variable transmission capable of changing the gearratio continuously, a belt-type continuously variable transmission and atoroidal type (traction-type) continuously variable transmission areknown in the art. The belt-type continuously variable transmission isadapted to transmit torque and change the gear ratio by using a belt,and the toroidal type continuously variable transmission is adapted totransmit torque and change the gear ratio by using a power roller. Inthe belt-type continuously variable transmission, the belt is woundaround drive and driven pulleys each capable of changing a groove width,and torque is transmitted by use of frictional force between contactsurfaces of the pulleys and the belt. With this arrangement, the gearratio of the CVT is changed by changing the groove width of the drivepulley so as to change an effective radius of the belt wound around thepulley.

In a toroidal continuously variable transmission, on the other hand, apower roller is sandwiched between an input disk and an output disk, andtorque is transmitted by use of shearing force of traction oil presentbetween the power roller and each of the disks. With this arrangement,the gear ratio of the CVT is changed by slanting or inclining therotating power roller to thereby change the radius of the position atwhich torque is transmitted between the power roller and each disk. Inthe continuously variable transmission of the above types, a torquetransmission portion takes the form of a surface, namely, torque istransmitted via surfaces of mutually facing members, so that the gearratio can be continuously changed.

As a power transmitting mechanism that transmits torque via surfaces, afriction clutch, a friction brake, and the like are known. Such afriction clutch or a friction brake is constructed such that the entireareas of frictional surfaces come in contact with and are spaced apartfrom each other, with the frictional surfaces being designed in view ofwears. A continuously variable transmission, on the other hand, isconstructed so as to transmit torque by bringing a belt or a powerroller into contact with a portion of a torque transmitting surface ofeach pulley or disk while continuously changing the torque transmittingportion. In such a continuously variable transmission, the torquetransmitting surface is designed without substantially allowing forwear, and therefore a local wear of the torque transmitting surface mayresult in poor torque transmission or a damage to the continuouslyvariable transmission.

Besides, there is a limit to the strength of constituent members orelements of the CVT, such as the belt, pulleys, disks, and traction oil.Therefore, the contact pressure between the corresponding members cannotbe increased without limit to avoid slippage of the continuouslyvariable transmission. Furthermore, when the contact pressure isincreased to a certain level, the efficiency of power transmission andthe durability of the continuously variable transmission may beundesirably reduced.

In continuously variable transmissions, therefore, the clamping forcefor clamping the belt or the power roller (or the load applied to clampthe belt or the power roller) is desired to be set to the minimum valuein a range that ensures that excessive slippage (so-called macro-slip)does not occur between the belt and the corresponding pulley or betweenthe power roller and the corresponding disk. Nevertheless, in general,the torque applied to the continuously variable transmissioncontinuously changes. Especially when a vehicle in which a continuouslyvariable transmission is used goes through a sudden acceleration orbrake, or is brought into a complicated operating state, such astemporary idling or slippage of drive wheels, a sudden and temporarilylarge torque may be applied to the continuously variable transmission.

If the clamping force is set to be a greater value in preparation forsuch temporarily large torque, the power transmitting efficiency and thefuel efficiency may deteriorate while the vehicle is running in normalor steady-state conditions. Accordingly, it is preferable to perform acontrol to increase the clamping force or reduce the torque applied tothe CVT when slippage due to large torque as described above is actuallydetected.

In the meantime, a system adapted for detecting a condition caused byslippage in a continuously variable transmission has been proposed inJapanese Laid-open Patent Publication No. 62-2059. The system disclosedin this publication is arranged to determine a failure or problem in thecontinuously variable transmission. In this system, rotation speeds of amain pulley and a sub-pulley are measured using sensors to calculate agear ratio. If the gear ratio thus measured or the rate of change in thegear ratio exhibits an extreme value that is not obtained in normalstate, the system is determined to be at faulty.

The sensors used in the system disclosed in the above publication arethe same as or equivalent to sensors generally used for a gear ratiocontrol of a continuously variable transmission. With the above systemthus constructed, therefore, a failure or a problem in the continuouslyvariable transmission can be detected without using other sensor(s)newly provided for this purpose. Thus, the system disclosed in the abovepublication is designed for determining a failure of the continuouslyvariable transmission, but is not provided, by nature, with any functionof dealing with excessive slippage of the belt.

That is, the system disclosed in the above publication is constructed soas to detect a failure of the continuously variable transmission for thefirst time when the gear ratio or the rate of change in the gear ratiotakes an abnormal value as a result of excessive slippage of the belt.The system, therefore, cannot be used for the purpose of avoiding aproblem caused by excessive slippage of the belt. In other words, thesystem disclosed in the above publication is not able to detect, withsufficiently high speed and accuracy, the beginning of a so-calledmacro-slip (i.e., considerably large slip) of the belt or a state thatmay lead to a macro-slip. Consequently, the above-described conventionalsystem cannot be used as a slippage detection system for performing acontrol for dealing with temporary slippage of the belt in thecontinuously variable transmission.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a system capableof immediately and accurately detecting slippage of a belt, or the like,in a continuously variable transmission, or detecting a start ofslippage or a state that may lead to slippage, without a dedicatedsensor or sensors.

To accomplish the above object, there is provided according to theinvention a slippage detection system for a continuously variabletransmission having an input member, an output member and a torquetransmitting member for transmitting torque between the input member andthe output member, the continuously variable transmission being capableof continuously changing a ratio between an input rotation speed of theinput member and an output rotation speed of the output member. Theslippage detection system includes (a) correlation coefficientcalculating means for calculating a correlation coefficient relating tothe input rotation speed and the output rotation speed, based on aplurality of measurement values of the input rotation speed and aplurality of measurement values of the output rotation speed, and (b)slippage determining means for determining slippage of the torquetransmitting member in the continuously variable transmission based onthe correlation coefficient calculated by the correlation coefficientcalculating means.

The slippage in the continuously variable transmission may be consideredas a state or condition in which the relationship between the input-siderotation speed and the output-side rotation speed deviates from apredetermined one that corresponds to a gear ratio (i.e., the ratio ofthe output-side rotation speed to the input-side rotation speed) to beestablished. The slippage detection system as described above calculatesa correlation coefficients that represents the relationship between theinput rotation speed and the output rotation speed, and is thereforeable to immediately and accurately determine a slipping state of theCVT, or a state that leads to excessive slippage, or a start ofexcessive slippage.

In one preferred embodiment of the invention, the slippage determiningmeans determines slippage of the torque transmitting member in thecontinuously variable transmission when the correlation coefficientcalculated by the correlation coefficient calculating means is smallerthan a predetermined reference value. The reference value may be setbased on an operating state of a vehicle in which the continuouslyvariable transmission is installed.

By setting the reference value to an appropriate value, the slippagedetection system is able to determine even a small degree of slippage aswell as large slippage, and perform suitable control to deal with theslippage. At the same time, the slippage detection system can avoidexcessively sensitive determination of slippage which would lead tounnecessary control for dealing with the slippage.

In another embodiment of the invention, the correlation coefficientcalculating means calculates the correlation coefficient when theoperating state of the vehicle in which the continuously variabletransmission is installed satisfies at least one predeterminedcondition. With this arrangement, even if the running state of thevehicle changes in a complicated manner, calculation of the correctioncoefficient is carried out only when the vehicle is in a suitablerunning state. It is thus possible to accurately determine a slippingstate in the continuously variable transmission.

In a further embodiment of the invention, the correlation coefficientcalculating means sets the number of the measurement values of each ofthe input rotation speed and the output rotation speed used forcalculating the correlation coefficient, based on an operating state ofa vehicle in which the continuously variable transmission is installed.

The above arrangement makes it possible to avoid erroneous determinationof slippage in the continuously variable transmission, or avoid asituation in which calculation of the correlation coefficient isunnecessarily repeated even if the running state of the vehicle does notchange.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or further objects, features and advantages of theinvention will become more apparent from the following description ofpreferred embodiments with reference to the accompanying drawings, inwhich like numerals are used to represent like elements and wherein:

FIG. 1 is a flowchart for explaining one example of control performed bya controller of a slippage detection system according to one embodimentof the invention;

FIG. 2 is a graph showing changes in a correlation coefficient withtime;

FIG. 3 is a graph schematically showing the trend or tendency of changesof reference values in accordance with the operating conditions of thevehicle;

FIG. 4 is a graph schematically showing the trend or tendency of changesof the number of sampling points for obtaining the correlationcoefficient in accordance with the operating conditions of the vehicle;

FIG. 5 is a flowchart for explaining another example of controlperformed by the slippage detection system according to the invention;

FIG. 6 is a graph showing changes in a band-pass value obtained bysubjecting an input-shaft rotation speed to a filtering process;

FIG. 7 is a flowchart for explaining another example of controlperformed by the slippage detection system according to the invention;

FIG. 8 is a graph showing changes in the accumulated band-pass value;

FIG. 9 is a flowchart for explaining another example of controlperformed by the slippage detection system according to the invention;

FIG. 10 is a graph showing changes in a difference between an actualgear ratio and a target gear ratio of a CVT;

FIG. 11 is another flowchart obtained by partially modifying theflowchart of FIG. 9;

FIG. 12 is a view showing changes in the sum of the differences;

FIG. 13 is another flowchart obtained by partially modifying theflowchart of FIG. 11; and

FIG. 14 is a view schematically showing a drive system and a controlsystem of a vehicle with a continuously variable transmission in which aslippage detection system according to the invention is employed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Some exemplary embodiments of the invention will be described in detail.First, a drive system and a control system of a motor vehicle to whichthe invention is applied will be described with reference to FIG. 14.FIG. 14 schematically shows a drive system including a belt-typecontinuously variable transmission (CVT) 1 as a transmission. The CVT 1is coupled to a power source 3 via a forward/reverse-drive switchingmechanism 2.

The power source 3 is a drive unit for generating power to run thevehicle, and is provided by an internal combustion engine, a combinationof an internal combustion engine and an electric motor, an electricmotor, or the like. In this embodiment, the power source 3 takes theform of an engine. The forward/reverse-drive switching mechanism 2 isemployed since the engine 3 can rotate only in one direction, and isarranged to output the input torque as it is or in a reverse direction.

In the example shown in FIG. 14, a double-pinion type planetary gearmechanism is used as the forward/reverse-drive switching mechanism 2. Inthis mechanism, a ring gear 5 is disposed concentrically with a sun gear4, and a pinion gear 6 that engages with the sun gear 4 and anotherpinion gear 7 that engages with the pinion gear 6 and the ring gear 5are disposed between the sun gear 4 and the ring gear 5. The piniongears 6, 7 are supported by a carrier 8 such that these gears 6, 7 arefreely rotatable about their center axes and about the center axis ofthe planetary gear mechanism. A forward-drive clutch 9 is provided forcoupling two rotating elements (i.e., sun gear 4 and carrier 8) into oneunit. Also, a reverse-drive brake 10 is provided for reversing thedirection of torque output from the forward/reverse-drive switchingmechanism 2 by fixing the ring gear 5 selectively.

The construction of the CVT 1 is the same as or equivalent to that of aknown belt-type continuously variable transmission. The CVT 1 isprovided with a drive pulley 11 and a driven pulley 12, which arearranged in parallel to each other. Each of the pulleys 11 and 12principally consists of a stationary sheave and a movable sheave that isadapted to be moved forward and backward in an axial direction thereofby a hydraulic actuator 13 or 14. With this arrangement, the groovewidth of each of the pulleys 11, 12 changes as the movable sheave of thepulley is moved in the axial direction thereof, thus continuouslychanging the winding radius of the belt 15 wound around the pulleys 11,12 (i.e., the effective radius of each pulley 11, 12) thereby tocontinuously change the gear ratio of the CVT 1. The drive pulley 11 isconnected to the carrier 8 serving as an output element of theforward/reverse-drive switching mechanism 2.

A hydraulic pressure (a line pressure or its corrected pressure) isapplied to the hydraulic actuator 14 for the driven pulley 12 via ahydraulic pump and a hydraulic control device or system (not shown). Thelevel of the hydraulic pressure applied to the hydraulic actuator 14 iscontrolled to be commensurate with the magnitude of the torque receivedby the CVT 1. With this arrangement, the belt 15 is clamped or grippedbetween the sheaves of the driven pulley 12 and is thus provided withsuitable tensile force, so that a suitable clamping force (or contactpressure) surely appears between each of the pulleys 11, 12 and the belt15. On the other hand, the hydraulic actuator 14 of the drive pulley 11is supplied with an oil pressure that depends on a desired gear ratio,thereby setting the groove width (or pitch diameter) of the pulley 11 toa target value.

The driven pulley 12 is connected to a differential gear unit 17 via apair of gears 16, and is adapted to output a torque to drive wheels 18via the differential gear unit 17.

Various sensors are provided for detecting operating conditions (orrunning conditions) of the vehicle including the CVT 1 and the engine 3.More specifically, there are provided an engine speed sensor 19 formeasuring the rotation speed of the engine 3 and generating a signalindicative of the engine speed, an input rotation speed sensor 20 formeasuring the rotation speed of the drive pulley 11 and generating asignal indicative of the input rotation speed, and an output rotationspeed sensor 21 for measuring the rotation speed of the driven pulley 12and generating a signal indicative of the output rotation speed. Inaddition, an accelerator position sensor, a throttle opening sensor, abrake sensor, and other sensors are provided, though not shown in thefigure. The accelerator position sensor is arranged to measure theamount of depression of an accelerator pedal and output a signalindicative of the accelerator pedal position. The throttle openingsensor is arranged to measure the opening amount of the throttle valveand output a signal indicative of the throttle opening. The brake sensoris arranged to output a signal when a brake pedal is depressed.

Also, an electronic control unit (CVT-ECU) 22 for transmission isprovided for controlling engagement and release of each of theforward-drive clutch 9 and the reverse-drive brake 10, the clampingforce applied to the belt 15, and the gear ratio of the CVT 1. Theelectronic control unit 22 for transmission includes, for example, amicrocomputer as its main component, and is arranged to performcalculations based on input data and data stored in advance, thereby toperform controls such as establishment of a selected operating mode,such as forward-drive, revere-drive or neutral mode, setting of therequired clamping pressure, and setting of the gear ratio of the CVT 1.

Input data (or signals) received by the electronic control unit 22 fortransmission may include, for example, signals indicative of input-shaftrotation speed Nin and output rotation speed No of the CVT 1 receivedfrom corresponding sensors (not shown). In addition, the electroniccontrol unit 22 for transmission receives signals indicative of enginespeed Ne, engine (E/G) load, throttle opening, accelerator position thatrepresents the amount of depression of the accelerator pedal (not shown)and so on, from an electronic control unit (E/G-ECU) 23 for controllingthe engine 3.

The CVT 1 is capable of continuously or steplessly controlling theengine speed as the input rotation speed as described above. When theCVT 1 is installed on a motor vehicle, therefore, the fuel efficiency ofthe vehicle is improved. For example, a target driving force isdetermined based on the required driving amount as represented by theaccelerator pedal position, or the like, and the vehicle speed. Then, atarget output of the CVT 1 needed for achieving the target driving forceis determined based on the target driving force and the vehicle speed.Then, an engine speed for achieving the target output with the optimumfuel efficiency is determined using a predetermined map. Finally, thegear ratio of the CVT 1 is controlled so as to achieve the determinedengine speed.

To make advantage of the improvement in the fuel efficiency, the powertransmitting efficiency of the CVT 1 is controlled to a desirably highlevel. More specifically, the torque capacity or the belt clampingpressure of the CVT 1 is controlled to be the minimum value in a rangein which the CVT 1 can transmit the target torque determined based onthe engine torque without causing slippage of the belt 15. This controlis normally performed in a steady state in which the vehicle speed andthe output requirement hardly change or in an almost steady state inwhich one or both of these parameters slightly changes.

Meanwhile, if the vehicle is suddenly braked or accelerated or if thevehicle runs upon a dropped object or a step, the torque applied to thedrive system including the CVT 1 suddenly changes. In this case, thetorque capacity of the CVT 1 may become relatively insufficient, thusincreasing the possibility of slippage of the belt 15. In such a case,therefore, the control system of the embodiment performs so-calledreactive control to temporarily increase the belt clamping force ortemporarily reduce the engine torque. The control system of theembodiment is arranged to perform the following control so as to judgeor determine occurrence of a situation (i.e., macro-slip) that requiresthe reactive control as described above.

FIG. 1 is a flowchart showing one example of the control for determininga macro-slip of the belt 15 of the CVT 1. In this control, a correlationcoefficient obtained based on the input and output rotation speeds isused. As shown in FIG. 1, it is first determined in step S1 whether therunning state of the vehicle is within a calculation range of thecorrelation coefficient. The correlation coefficient used in thiscontrol is a coefficient calculated based on the input-shaft rotationspeed (xi) and the output-shaft rotation speed (yi). When each of theinput and output rotation speeds has a value other than 0 and the gearratio is kept almost constant, the running state of the vehicle isdetermined to be within the calculation range of the correlationcoefficient. That is, the correlation coefficient is within thecalculation range when the vehicle is running while the gear ratio iskept almost constant (i.e., the speed ratio, which is the inverse of thegear ratio, is kept almost constant).

If a negative determination is made in step S1, a flag F is reset to 0in step S2 and the control returns. If a positive determination is madein step S1, on the other hand, the flag F is set to 1 in step S3 and theinput-shaft rotation speed (xi) and the output-shaft rotation speed (yi)are read in steps S4 and S5, respectively. These rotation speeds (xi,yi) are respectively measured by the input rotation speed sensor 20 andthe output rotation speed sensor 21 shown in FIG. 14. In step S6, acorrelation coefficient S is obtained using N sets of the rotationspeeds (xi, yi) that have been read so far.

The correlation coefficient S is represented by the expression (1)below:

$\begin{matrix}{{{correlation}\mspace{14mu} {coefficient}\mspace{14mu} S} = \frac{{x_{1} \cdot y_{1}} + {x_{2} \cdot y_{2}} + \ldots + {x_{n} \cdot y_{n}}}{\begin{matrix}\sqrt{x_{1}^{2} + x_{2}^{2} + \ldots + x_{n}^{2}} \\\sqrt{y_{1}^{2} + y_{2}^{2} + \ldots + y_{n}^{2}}\end{matrix}}} & (1)\end{matrix}$

In the above expression (1), each suffix (1, 2 . . . n) represents asampling point at which the rotation speed (xi or yi) was measured and nrepresents the present time.

An occurrence or possibility of slippage (macro-slip) of the belt isdetermined by using the correlation coefficient S in the followingmanner. In the expression (1), the power of the rotation speeds of theinput and output members (i.e., the input and output shafts of the CVT1) is normalized by the square root of the powers of the input rotationspeed and the output rotation speed. According to the expression (1),when the power of the input and output rotation speeds decreases, thenormalized value decreases. More specifically, when slippage of the belt15 does not occur, the correlation coefficient S is equal to 1. Whenslippage of the belt 15 occurs, conversely, the value becomes smallerthan 1.

Thus, while the belt 15 is not slipping but is being gripped by thedrive and driven pulleys 11, 12, the relationship as represented byexpression (2) below is true:

yi=γ·xi   (2)

Here, γ represents the speed ratio (which is the inverse of the gearratio).

When the expression (2) is assigned to the above expression (1), thecorrelation coefficient S is represented by expression (3), and itsvalue becomes 1.

$\begin{matrix}\begin{matrix}{{{correlation}\mspace{14mu} {coefficient}\mspace{14mu} S} = \frac{\gamma \cdot \left( {{x_{1} \cdot x_{1}} + {x_{2} \cdot x_{2}} + \ldots + {x_{n} \cdot x_{n}}} \right)}{\begin{matrix}\sqrt{x_{1}^{2} + x_{2}^{2} + {\ldots \mspace{20mu} x_{n}^{2}}} \\\sqrt{\gamma^{2} \cdot \left( {x_{2}^{2} + {\ldots \mspace{14mu} x_{n}^{2}}} \right)}\end{matrix}}} \\{= \frac{\gamma \cdot \left( {x_{1}^{2} + x_{2}^{2} + \ldots + x_{n}^{2}} \right)}{\gamma \cdot \left( {x_{1}^{2} + x_{2}^{2} + \ldots + x_{n}^{2}} \right)}} \\{= 1.0}\end{matrix} & (3)\end{matrix}$

As described above, one of the conditions for the calculation is thatthe speed ratio γ be almost constant in order to put the speed ratio γout of the parentheses. It is thus not preferable to measure theinput-shaft and output-shaft rotation speeds at long time intervals orsampling time.

Next, there will be described the case where the belt 15 is not beingsufficiently gripped by the drive and driven pulleys 11, 12 and isslipping. While the belt 15 is slipping, the relationship between theinput-shaft rotation speed (xi) and the output-shaft rotation speed (yi)with respect to the speed ratio γ that is currently set becomes untrue.The relationship between these rotation speeds is then represented inexpression (4) below:

yi=ki·γ·xi   (4)

Here, ki, which is a real number larger than “0”, is a coefficientrepresenting rotational fluctuations or variations.

In this case, the expression (4) is assigned to the above expression(1), and the correlation coefficient S is then represented by thefollowing expression (5):

$\begin{matrix}\begin{matrix}{{{correlation}\mspace{14mu} {coefficient}\mspace{14mu} S} = \frac{\gamma \cdot \begin{pmatrix}{{k_{1} \cdot x_{1} \cdot x_{1}} + {k_{2} \cdot}} \\{x_{2} + \ldots + {k_{n} \cdot x_{n} \cdot x_{n}}}\end{pmatrix}}{\begin{matrix}\sqrt{x_{1}^{2} + x_{2}^{2} + \ldots \; + x_{n}^{2}} \\\sqrt{\gamma^{2} \cdot \begin{pmatrix}{{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot}} \\{x_{2}^{2} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}}\end{pmatrix}}\end{matrix}}} \\{= \frac{{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}}{\begin{matrix}\sqrt{x_{1}^{2} + x_{2}^{2} + \ldots \; + x_{n}^{2}} \\\sqrt{\begin{pmatrix}{{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot}} \\{x_{2}^{2} + {\ldots \mspace{14mu} {k_{n}^{2} \cdot x_{n}^{2}}}}\end{pmatrix}}\end{matrix}}}\end{matrix} & (5)\end{matrix}$

When the coefficient ki is not constant due to the rotationalfluctuations caused by slippage of the belt 15, the correlationcoefficient S becomes smaller than 1. Namely, the expression (5) istransformed into the following expression (6).

$\begin{matrix}{{{correlation}\mspace{14mu} {coefficient}\mspace{14mu} S} = \frac{\sqrt{\left( {{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}} \right)^{2}}}{\begin{matrix}\sqrt{x_{1}^{2} + x_{2}^{2} + \ldots \; + x_{n}^{2}} \\\sqrt{\begin{pmatrix}{{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot}} \\{x_{2}^{2} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}}\end{pmatrix}}\end{matrix}}} & (6)\end{matrix}$

When the numerator and denominator of the expression (6) are expanded,the following expressions (7) and (8) will be provided respectively:

$\begin{matrix}{{{\left( {{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}} \right) \cdot \left( {{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}} \right)} = {{k_{1} \cdot x_{1}^{2} \cdot \left( {{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}} \right)} + {k_{2} \cdot x_{2}^{2} \cdot \left( {{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}} \right)}}}{{\vdots  + {k_{n} \cdot x_{n}^{2} \cdot \left( {{k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot x_{2}^{2}} + \ldots + {k_{n} \cdot x_{n}^{2}}} \right)}} = {{k_{1}^{2} \cdot x_{1}^{4}} + {k_{2}^{2} \cdot x_{2}^{4}} + \ldots + {k_{n}^{2} \cdot x_{n}^{4}} + {x_{1}^{2} \cdot \left( {{k_{1} \cdot k_{2} \cdot x_{2}^{2}} + \ldots + {k_{1} \cdot k_{n} \cdot x_{n}^{2}}} \right)} + {x_{2}^{2} \cdot \left( {{k_{2} \cdot k_{1} \cdot x_{1}^{2}} + \ldots + {k_{2} \cdot k_{n} \cdot x_{n}^{2}}} \right)}}}{\vdots  + {x_{n}^{2} \cdot \left( {{k_{n} \cdot k_{1} \cdot x_{1}^{2}} + \ldots + {k_{n} \cdot k_{n - 1} \cdot x_{n - 1}^{2}}} \right)}}} & (7) \\{{{\left( {x_{1}^{2} + x_{2}^{2} + \ldots + x_{n}^{2}} \right) \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot x_{2}^{2}} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}} \right)} = {{x_{1}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot x_{2}^{2}} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}} \right)} + {x_{2}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot x_{2}^{2}} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}} \right)}}}{\vdots  + {x_{n}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot x_{2}^{2}} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}} \right)}} = {{k_{1}^{2} \cdot x_{1}^{4}} + {k_{2}^{2} \cdot x_{2}^{4}} + \ldots + {k_{n}^{2} \cdot x_{n}^{4}} + {x_{1}^{2} \cdot \left( {{k_{2}^{2} \cdot x_{2}^{2}} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}} \right)} + {{x_{2}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + \ldots + {k_{n}^{2} \cdot x_{n}^{2}}} \right)}\vdots} + {x_{n}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {\ldots \mspace{14mu} {k_{n - 1}^{2} \cdot x_{n - 1}^{2}}}} \right)}}} & (8)\end{matrix}$

If the sampling time n is 3, the expressions (7) and (8) are rewritteninto the following expressions (9) and (10), respectively:

$\begin{matrix}{{{k_{1}^{2} \cdot x_{1}^{4}} + {k_{2}^{2} \cdot x_{2}^{4}} + k_{3}^{2} + x_{3}^{4} + {x_{1}^{2} \cdot \left( {{k_{1} \cdot k_{2} \cdot x_{2}^{2}} + {k_{1} \cdot k_{3} \cdot x_{3}^{2}}} \right)} + {x_{2}^{2} \cdot \left( {{k_{2} \cdot k_{1} \cdot x_{1}^{2}} + {k_{2} \cdot k_{3} \cdot x_{3}^{2}}} \right)} + {x_{3}^{2} \cdot \left( {{k_{3} \cdot k_{1} \cdot x_{1}^{2}} + {k_{3} \cdot k_{2} \cdot x_{2}^{2}}} \right)}} = {{k_{1}^{2} \cdot x_{1}^{4}} + {k_{2}^{2} \cdot x_{2}^{4}} + {k_{3}^{2} \cdot x_{3}^{4}} + \left( {{2 \cdot k_{1} \cdot k_{2} \cdot x_{1}^{2} \cdot x_{2}^{2}} + {2 \cdot k_{1} \cdot k_{3} \cdot x_{1}^{2} \cdot x_{3}^{2}} + {2 \cdot k_{2} \cdot k_{3} \cdot x_{2}^{2} \cdot x_{3}^{2}}} \right)}} & (9) \\{{{k_{1}^{2} \cdot x_{1}^{4}} + {k_{2}^{2} \cdot x_{2}^{4}} + {k_{3}^{2} \cdot x_{3}^{4}} + {x_{1}^{2} \cdot \left( {{k_{2}^{2} \cdot x_{2}^{2}} + {k_{3}^{2} \cdot x_{3}^{2}}} \right)} + {x_{2}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {k_{3}^{2} \cdot x_{3}^{2}}} \right)} + {x_{3}^{2} \cdot \left( {{k_{1}^{2} \cdot x_{1}^{2}} + {k_{2}^{2} \cdot x_{2}^{2}}} \right)}} = {{k_{1}^{2} \cdot x_{1}^{4}} + {k_{2}^{2} \cdot x_{2}^{4}} + {k_{3}^{2} \cdot x_{3}^{4}} + \left( {{x_{1}^{2} \cdot x_{2}^{2} \cdot \left( {k_{1}^{2} + k_{2}^{2}} \right)} + {x_{1}^{2} \cdot x_{3}^{2} \cdot \left( {k_{1}^{2} + k_{3}^{2}} \right)} + {x_{2}^{2} \cdot x_{3}^{2} \cdot \left( {k_{2}^{2} + k_{3}^{2}} \right)}} \right)}} & (10)\end{matrix}$

When the respective coefficients affixed to x₁ ²·x₂ ², or the like, inthe expressions (9) and (10) are compared, the relationship asrepresented by the following expression (11) is found to be true:

k _(j) ² +k _(m) ²≧2·k _(j) ·k _(m)   (11)

Here, j and m are suffixes such as 1 and 2.

The expression (11) may be rewritten into the following expression (12):

(k _(j) −k _(m))²≧0   (12)

Here, since Ki and Km are real numbers, the relationship of theexpression (12) is always true and therefore the relationship of theexpression (11) is also true. Also, when the sampling number n is largerthan 3, the relationships of the expressions (11) and (12) are true.When the input/output rotation speeds start varying, the value of thedenominator as described above becomes larger than that of thenumerator. As a result, the correlation coefficient S becomes smallerthan 1. Accordingly, it is possible to determine an occurrence ofslippage of the belt 15 based on the correlation coefficient S.

In step S6 of FIG. 1, the correlation coefficient S is determinedthrough calculations. In step S7, it is determined whether thecorrelation coefficient S is equal to or smaller than a first referencevalue S1 determined in advance. The first reference value S1 is smallerthan 1 and is determined in advance as a value corresponding to a statein which a macro-slip is occurring or a slipping state which may lead toa macro-slip.

If the correlation coefficient S is equal to or smaller than the firstreference value S1 and a positive determination is therefore made instep S7, an occurrence or a possibility of a macro-slip is thendetermined (macro-slip determination is made) in step S8. In the nextstep S9, a responsive control is performed in response to the macro-slipdetermination made in step S8. In short, the responsive control isperformed to avoid or suppress macro-slips. As the responsive control,for example, the clamping force applied to the belt 15 is increased orthe engine torque is reduced. In addition, if a clutch is used fortransmitting torque to the CVT 1, for example, the capacity of theclutch is reduced under the responsive control. The degree of theresponsive control is set in accordance with the degree of themacro-slip, namely, the value of the correlation coefficient S.

Conversely, when the correlation coefficient S is larger than the firstreference value S1 described above, and a negative determination is madein step S7, it is then determined in step S10 whether the correlationcoefficient S is equal to or larger than a second reference value S2.The second reference value S2 is a value larger than the first referencevalue S1 but smaller than 1 and is determined in advance as a value thatcorresponds to a state where a relatively small macro-slip is occurringor a state that may lead to such a relatively small macro-slip.

When a negative determination is made in step S10, namely, when thecorrelation coefficient S is smaller the second reference value S2, itindicates that a relatively small macro-slip is occurring or is highlylikely to occur. In such a case, therefore, the control proceeds to stepS8 to make a macro-slip determination. Subsequently, the responsivecontrol is performed in step S9.

When a negative determination is made in step S10, conversely, anon-macro-slip determination is made in step S11. The non-macro-slipdetermination is made when a macro-slip is not occurring or is notlikely to occur, or when the belt 15 is slipping but the degree ofslipping is within a permissible range. In this case, responsive controlis performed as needed in step S12 in response to the non-macro-slipdetermination made in step S11. One example of the responsive control isa control for reducing the clamping force applied to the belt 15. Thiscontrol is intended to improve the power transmitting efficiency of theCVT 1 and to minimize the hydraulic pressure supplied to the CVT 1 tothereby reduce power loss at the hydraulic pump, thus assuring improvedfuel efficiency.

FIG. 2 is a graph showing changes in the correlation coefficient Sduring a transition from a state where a macro-slip of the belt 15 isnot occurring to a state where a macro-slip of the belt 15 is occurring.When torque is being transmitted via the CVT 1, slippage, which can bedenominated “micro-slip” in contrast to “macro-slip”, unavoidablyoccurs. Thus, the torque is transmitted via the CVT 1 with thecorrelation coefficient S changing extremely slightly. When an operatingstate of the vehicle that leads to a macro-slip of the belt 15 takesplace for some reason, the correlation coefficient S starts decreasingto some extent. When a macro-slip subsequently starts, the correlationcoefficient S starts decreasing rapidly. For example, the referencevalues S1 and S2 are respectively set to the values as indicated in FIG.2.

In the meantime, the correlation coefficient S is determined based onthe input-shaft and output-shaft rotation speeds. The relationshipbetween these rotation speeds change not only due to slippage of thebelt 15 but also due to changes in the engine torque,acceleration/deceleration of the vehicle, and the like, which are causedwhen the accelerator operation amount changes. While the correlationcoefficient S may decrease due to these changes, such a decrease in thecorrelation coefficient S does not indicate an occurrence or apossibility of a macro-slip. In this case, therefore, there is a need toavoid making the macro-slip determination. To meet this need, each ofthe reference values S1 and S2 may be changed in accordance with theoperating conditions of the vehicle, such as the rate of change of theaccelerator operation amount, acceleration/deceleration of the vehicle,and so on.

FIG. 3 is a graph showing one example of the tendency of changes in thereference values S1, S2. As shown in FIG. 3, the respective referencevalues S1, S2 are reduced as the rate of change of the acceleratoroperation amount ΔACC or the acceleration/deceleration ΔV increases.Thus, if the correlation coefficient S decreases due to a factor orfactors other than a macro-slip, an occurrence or a possibility of amacro-slip will not be determined by mistake, and a responsive controlwill not be performed by mistake in response to the decrease in thecorrelation coefficient S. Thus, erroneous determination of a macro-slipis avoided, and unnecessary responsive control can also be avoided orsuppressed. Furthermore, a delay in determining an occurrence or apossibility of slippage is prevented.

Also, the correlation coefficient S is calculated using a plurality ofdetection values representing the input and output rotation speeds. Thenumber of sets of the detected values (which will be referred to as “setnumber”) is preferably determined in accordance with the operating stateof the vehicle. FIG. 4 is a graph schematically showing one example ofthe tendency or trend in determining the set number N. As shown in FIG.4, the set number N is reduced as the vehicle speed V, theacceleration/deceleration ΔV, the rate of change in the acceleratoroperation amount ΔACC, the gear ratio γ, or the like, increases. Whenthe rate of change of the accelerator operation amount is large, forexample, the degree or magnitude of a corresponding change in the gearratio is supposed to be large. In this case, the set number is reducedin order to prevent the correlation coefficient S from being calculatedbased on the rotation speeds at largely different gear ratios andthereby prevent erroneous determination of a macro-slip and a delay indetermining an occurrence or a possibility of slippage of the belt 15.

As described above, the slippage detection system of the embodiment isarranged to determine macro-slips based on the correlation coefficientby measuring the input and output rotation speeds by means of thesensors 21, 22 that are normally used for determining the gear ratio ofthe CVT 1. With this arrangement, slippage of the belt 15 can beimmediately determined with sufficiently high accuracy without usingother sensor or sensors dedicated to this function. Moreover, since theslippage detection system is able to carry out necessary responsivecontrol based on the determination of macro-slips, a damage to the CVT 1as a result of excessive slippage of the belt 15 can be prevented orsuppressed.

Meanwhile, the input-shaft rotation speed Nin of the CVT 1 changes dueto various factors, such as the gear ratio control, slippage of the belt15, or periodical variations in the input torque. Therefore, bydetermining the amount of change of the input rotation speed that iscaused by slippage of the belt 15, out of the overall change amount, itis possible to determine an occurrence or a possibility of a macro-slipof the belt 15 based on the determined value (change amount). Oneexample of such control will be described in the following.

FIG. 5 is a flowchart showing one example of the control. In thiscontrol, the input-shaft rotation speed Nin detected by the inputrotation speed sensor 20 is first read in step S21. Next, a vibrationcomponent Nin·vib contained in the input-shaft rotation speed Nin andresulting from slippage of the belt 15 is obtained. Here, the vibrationcomponent Nin·vib can be obtained, for example, by carrying out aband-pass filtering process or on the basis of a deviation of the actualinput-shaft rotation speed Nin from a command value of the input-shaftrotation speed. The command value is determined so as to achieve thedesired gear ratio. During the band-pass filtering process, measurementnoises are also removed.

FIG. 6 is a graph showing one example of changes in the band-pass valuewith time when the input-shaft rotation speed Nin is subjected toband-pass filtering (20-30 Hz). When a macro-slip does not occur asshown in FIG. 6, the band-pass value stays in a relatively small range.When a macro-slip takes place, on the other hand, the band-pass valuerapidly increases. In view of this, a reference value Nin·vib1 isdetermined in advance as an index or criteria for determining anoccurrence of a macro-slip, and it is determined in step S23 whether thevibration component Nin·vib obtained in step S22 is equal to or largerthan the reference value Nin·vib1. Meanwhile, the reference valueNin·vib1 may be changed in accordance with the operating state of thevehicle, rather than being constant, so as to prevent erroneousdeterminations from being made and also avoid a delay in determining anoccurrence of a macro-slip.

If the vibration component Nin·vib is equal to or larger than thereference value Nin·vib1 and a positive determination is therefore madein step S23, the macro-slip determination is made in step S24, and aresponsive control is performed in step S25. The operations in step S24and step S25 are the same as or equivalent to those in step S8 and stepS9 of FIG. 1, respectively.

If the vibration component Nin·vib is smaller than the reference valueNin·vib1 and a negative determination is therefore made in step S23, itindicates that a macro-slip is not occurring as is understood from FIG.6. In this case, a non-macro-slip determination is made in step S26.Subsequently, normal control is performed in step S27. In this normalcontrol, the belt clamping force is set in accordance with, for example,the engine torque or the amount of depression of the accelerator pedal(i.e., accelerator operation amount).

In order to perform the control as illustrated above with reference toFIGS. 5 and 6, the slippage detection system of the embodiment uses onlythe input rotation speed sensor 20 as a sensor so as to immediately andaccurately determine macro-slips of the belt 15 without requiring othersensor or sensors for this purpose. Moreover, since the slippagedetection system is able to perform required responsive control upondetection of a macro-slip, an otherwise possible damage to the CVT 1 asa result of excessive slippage of the belt 15 can be prevented.

While an occurrence of a macro-slip is determined based on the band-passvalues in the control illustrated in FIG. 5, the slippage detectionsystem according to another embodiment of the invention is constructedso as to determine an occurrence of a macro-slip based on theaccumulated value of the vibration components due to slippage of thebelt during a period from a previous point of time to the current pointof time. One example of such control will be described in the following.

In this control, as shown in FIG. 7, the input-shaft rotation speed Ninis read in step S31 and the vibration component Nin·vib is determined instep S32 in the same manner as in steps S21 and step S22 of FIG. 5,respectively. Subsequently, it is determined in step S33 whether it ispossible to carry out accumulation of the vibration components Nin·vib.More specifically, it is determined in step S33 whether i sets of datarequired for executing a time-window accumulation of the vibrationcomponents have been obtained.

If a negative determination is made in step S33, the control returns,and waits for a required number of data sets to be obtained. If apositive determination is made in step S33, conversely, step S34 isexecuted to calculate a time-window accumulation value S−vib(N) of thevibration components obtained during a period between the present time(N time point) and a previous point of time (N−1 time point) that is apredetermined time prior to the present time. Here, the number of datasets to be accumulated or the time period during which the data areaccumulated may be changed depending upon the operating state of thevehicle.

FIG. 8 is a graph showing changes in the time-window accumulation valueof the vibration components Nin−vib resulting from slippage of the belt15. When a macro-slip does not occur, the accumulated band-pass valueS−vib(N) is held within a relatively small range, as shown in FIG. 8.When a macro-slip occurs, on the other hand, the accumulated band-passvalue S−vib(N) increases rapidly. In view of this, a reference value Sa,which is used as a criteria or threshold for determining an occurrenceof a macro-slip, is set in advance, and it is determined in step S35whether the accumulated band-pass value S−vib(N) obtained in step S34 isequal to or larger than the reference value Sa. Meanwhile, the referencevalue Sa may be changed in accordance with the operating state of thevehicle, rather than being constant, so as to prevent an erroneousdetermination of a macro-slip and also avoid a delay in determining anoccurrence of a macro-slip.

If the accumulated band-pass value S−vib(N) is equal to or larger thanthe reference value Sa and a positive determination is made in step S35,the macro-slip determination is made in step S36 and a responsivecontrol is then performed in step S37. These operations in steps S36 andS37 are the same as or equivalent to those in step S24 and step S25 ofFIG. 5, or those in step S8 and step S9 of FIG. 1, respectively.

If the accumulated band-pass value S−vib(N) is smaller than thereference value Sa and a negative determination is made in step S35, onthe other hand, it indicates that a macro-slip is not occurring as isunderstood from FIG. 8. In this case, therefore, a non-macro-slipdetermination is made in step S38. Subsequently, normal control isperformed in step S37. These operations in step S38 and step S39 are thesame as or equivalent to those in step S26 and step S27 of FIG. 5,respectively.

In order to perform the control as illustrated above with reference toFIGS. 7 and 8, the slippage detection system of the embodiment uses onlythe input rotation speed sensor 20 as a sensor to immediately andaccurately determine macro-slips of the belt 15 without requiring othersensor or sensors for this purpose. Moreover, since the slippagedetection system is able to perform required responsive control upondetection of a macro-slip, an otherwise possible damage to the CVT 1 asa result of excessive slippage of the belt 15 can be prevented.

As described above, slipping of the belt 15 causes changes in the inputand output rotation speeds. With the rotation speeds thus changed, theactual gear ratio, which is obtained as a ratio between the input-shaftrotation speed and the output-shaft rotation speed, deviates from a gearratio (i.e., a target gear ratio) established immediately before theoccurrence of slippage of the belt 15, resulting in a difference betweenthe actual gear ratio and the target gear ratio. According to anotherembodiment of the invention, an occurrence of a macro-slip is determinedon the basis of the above-described difference between the actual gearratio and the target gear ratio.

FIG. 9 is a flowchart showing one example of control for determining amacro-slip in the above-described manner. In this control, it is firstdetermined in step S41 whether the gear ratio is being changed, namely,the CVT 1 is in the middle of a shifting action. The target gear ratiois generally set on the basis of the output requirement (e.g.,accelerator operation amount) and the vehicle speed or engine speed, forexample. When the CVT 1 is in the middle of a shifting action, however,the target gear ratio or the target input rotation speed correspondingto the target gear ratio may be set as a value with a first-order lagwith respect to a finally set value. Accordingly, the varying targetgear ratio cannot be used as a basis for determining an occurrence or apossibility of slippage of the belt 15. Accordingly, in step S41, it isdetermined whether the CVT 1 is in the middle of a shifting action, andif a positive determination is made in step S41, the control returnswithout performing any particular control.

If the CVT 1 is not in the middle of a shifting action and a negativedetermination is made in step S41, the actual gear ratio y is calculatedin step S42 as a ratio between the input rotation speed Nin and theoutput rotation speed Nout, both obtained through actual measurements.Subsequently, a target gear ratio γtag is calculated in step S43 as aratio between the target input rotation speed Nint and the outputrotation speed Nout obtained through an actual measurement. Then, it isdetermined in step S44 whether an absolute value of a difference betweenthe actual gear ratio γ and the target gear ratio γtag is larger than areference value Δγa that has been determined in advance.

FIG. 10 is a graph showing one example of a situation where thedifference between the actual gear ratio γ and the target gear ratioγtag changes. Since the input rotation speed changes due to variousfactors while the CVT 1 is being operated, as described above, the gearratio difference between the actual and target gear ratios keeps varyingslightly in the positive and negative directions with respect to zero asshown in FIG. 10. When a macro-slip is not occurring, the gear ratiodifference is maintained in a small range. When a macro-slip occurs,however, the input rotation speed starts deviating largely from thetarget value, resulting in an increase in the gear ratio difference.Accordingly, it is possible to determine an occurrence or a possibilityof a macro-slip by determining whether the gear ratio difference issmaller or larger than a threshold value established for determinationof macro-slips.

More specifically, in the example as illustrated in FIG. 9, the numberof times the gear ratio difference exceeds the reference value Δγa iscounted in step S45. Subsequently, it is determined in step S46 whetherthe above number of times the above condition (|γ−γtag|>Δγa) of step S44is satisfied has reached a predetermined number within a predeterminedperiod of time. This determination is made so as to prevent an erroneousdetermination due to disturbances such as noise.

If a positive determination is made in step S46, an occurrence or apossibility of slippage, or a macro-slip, of the belt 15 is determinedin step S47. In this case, the slippage detection system performs acontrol in response to the detected macro-slip, such as increasing thebelt clamping force or reducing the engine torque, as in the respectiveexamples of control as described above. If a negative determination ismade in step S46, conversely, the control returns.

On the other hand, if the gear ratio difference is equal to or smallerthan the reference value Δγa and a negative determination is made instep S44, it is then determined in step S48 whether this state haslasted for a predetermined period of time. If a negative determinationis made in step S48, the control returns, thus waiting for time to pass.When a positive determination is made in step S48, on the other hand, itindicates that the actual gear ratio γ is not largely different from thetarget gear ratio γtag and this situation has lasted for thepredetermined period. In this case, the slippage determination iscanceled in step S49.

In order to perform the control as illustrated above with reference toFIG. 9, the slippage detection system of the embodiment uses only theinput rotation speed sensor 20 as a sensor to immediately and accuratelydetermine macro-slips of the belt 15 without requiring other sensor orsensors for this purpose. Moreover, since the slippage detection systemis able to perform required responsive control upon detection of amacro-slip, an otherwise possible damage to the CVT 1 as a result ofexcessive slippage of the belt 15 can be prevented.

In the control as illustrated in FIG. 9, the number of times the gearratio difference exceeds the reference value Δγa is counted fordetermining an occurrence or a possibility of slippage as describedabove. Instead, the sum of gear ratio differences that have beenaccumulated for a predetermined period of time or at a predeterminednumber of sampling points may be used for determining an occurrence or apossibility of slippage. More specifically, the number of times the sumof the gear ratio differences accumulated as described above exceeds apredetermined reference value sumγ is counted. If the number of timeshas reached a predetermined number within a predetermined period, it isdetermined that slippage is occurring. FIG. 11 is a flowchart showingone example of control for determining an occurrence or a possibility ofa macro-slip in such a manner. The operations in respective steps of theflowchart illustrated in FIG. 11 are the same as those in the flowchartillustrated in FIG. 9 except that step S44 of FIG. 9 is replaced by stepS44A of FIG. 11. Meanwhile, the reference value sumγ may be changed inaccordance with the operating state of the vehicle, rather than beingconstant, so as to prevent an erroneous determination or a delay indetermining an occurrence or a possibility of a macro-slip.

FIG. 12 is a graph showing changes in the sum of the gear ratiodifferences that are added up with respect to a predetermined number of(e.g., ten) sampling points shown in FIG. 10. As is apparent from FIG.12, when a macro-slip does not occur, the sum of the gear ratiodifferences is maintained at relatively small values. When a macro-slipoccurs, on the other hand, the sum begins to increase rapidly.Accordingly, it is possible to determine an occurrence or a possibilityof a macro-slip when the sum exceeds a predetermined threshold value.Alternatively, it is also possible to determine an occurrence or apossibility of a macro-slip based on the number of times the sum exceedsthe reference value sumγ, rather than merely comparing the sum with thethreshold value, so that an erroneous determination due to some type ofdisturbance can be prevented or avoided.

In order to perform the control as illustrated above with reference toFIG. 11, the slippage detection system of the embodiment uses only theinput rotation speed sensor 20 as a sensor to immediately and accuratelydetermine macro-slips of the belt 15 without requiring other sensor orsensors for this purpose. Moreover, since the slippage detection systemis able to perform required responsive control upon detection of amacro-slip, an otherwise possible damage to the CVT 1 as a result ofexcessive slippage of the belt 15 can be prevented.

As is apparent from FIG. 10 or 12 showing changes in the gear ratiodifference or the sum of gear ratio differences, once the belt 15 startsslipping (i.e., a macro-slip appears), these values continue to increaseprogressively, and are maintained at large values until, for example,the CVT 1 breaks and stops operating, or the belt clamping force isextremely increased, or the engine torque is extremely reduced.Accordingly, an occurrence or a possibility of a macro-slip may bedetermined based on a time duration for which the sum of gear ratiodifferences is kept larger than the reference value sumγ, instead ofcounting the number of times the sum exceeds the reference value sumγ.

FIG. 13 is a flowchart showing one example of control for determining anoccurrence or a possibility of a macro-slip in such a manner. Theoperations in respective steps of the flowchart illustrated in FIG. 13are the same as those in the flowchart illustrated in FIG. 11, exceptthat steps S45 and S46 in the flowchart of FIG. 11 are replaced by stepS45A of the flowchart of FIG. 13. In step S45A, it is determined whetherthe condition determined in step S44A has been continuously satisfiedfor a predetermined period of time.

In order to perform the control as illustrated above, the slippagedetection system of the embodiment uses only the input rotation speedsensor 20 as a sensor to immediately and accurately determinemacro-slips of the belt 15 without requiring other sensor or sensors forthis purpose, as in the case of control of FIG. 11. Moreover, since theslippage detection system is able to perform required responsive controlupon detection of a macro-slip, an otherwise possible damage to the CVT1 as a result of excessive slippage of the belt 15 can be prevented.

While the determination of belt slippage (macro-slip) is not performedduring a shifting action of the CVT 1 in the control routines asillustrated in FIGS. 9, 11, and 13, the slippage detection system may beconstructed so as to perform the determination of belt slippage evenwhen the CVT 1 is in the middle of a shifting action. In this case,however, the target input rotation speed is set to a large value and thedifference between the actual gear ratio and the target gear ratiobecomes large, which may result in a deterioration in the accuracy indetermining belt slippage. When performing the determination of beltslippage by using the gear ratio difference during a shifting action ofthe CVT 1, therefore, it is preferable to subject the target inputrotation speed to a smoothing operation, and calculate the gear ratiodifference using a target gear ratio determined based on the smoothenedtarget input rotation speed. In this manner, an erroneous determinationon an occurrence or a possibility of a macro-slip can be suppressed oravoided.

While the slippage detection systems of the illustrated embodiments ofthe invention are adapted for use in belt-type CVTs, the invention maybe applied to slippage detection systems for use in toroidal-type(traction-type) continuously variable transmissions. Furthermore, theinput rotation speed is not limited to the rotation speed of the inputshaft. More specifically, the input rotation speed may be defined as therotation speed of any member of the continuously variable transmissionthat is arranged to rotate by receiving torque from a power source, orthe rotation speed of any member provided as a unit with that member. Inthe same way, the output rotation speed is not limited to the rotationspeed of the output shaft. More specifically, the output rotation speedmay be defined as the rotation speed of any member of the continuouslyvariable transmission that is arranged to rotate with torque transmittedfrom an input-side member or the rotation speed of any member providedas a unit with that member. Still further, the slippage detection systemaccording to the invention may be constructed so as to perform aplurality of the above-described slippage determination controls incombination.

1. A slippage detection system for a continuously variable transmissioncapable of continuously changing a gear ratio between an input rotationspeed of an input member and an output rotation speed of an outputmember, comprising: a sum calculator configured to calculate a sum ofdifferences between an actual gear ratio calculated from measurementvalues of the input rotation speed and the output rotation speed and atarget gear ratio over a predetermined period of time; a slippagedetermining portion configured to determine slippage in the continuouslyvariable transmission based on the sum of differences calculated by thesum calculating means.
 2. The slippage detection system according toclaim 1, wherein: the continuously variable transmission includes theinput member, the output member, and a torque transmitting member fortransmitting torque between the input member and the output member; andthe slippage determining portion determines slippage of the torquetransmitting member in the continuously variable transmission.
 3. Theslippage detection system according to claim 1, wherein the slippagedetermining portion determines slippage in the continuously variabletransmission based on the number of times the sum of differences exceedsa predetermined reference value within a predetermined period of time.4. The slippage detection system according to claim 1, wherein theslippage determining portion determines slippage in the continuouslyvariable transmission based on a period of time for which the sum ofdifferences is kept larger than a predetermined reference value.
 5. Theslippage detection system according to claim 1, wherein the target gearratio is determined based on a value obtained as a result of a smoothingoperation performed on a target input rotation speed that is determinedbased on an output requirement of a vehicle in which the continuouslyvariable transmission is installed.
 6. The slippage detection systemaccording to claim 1, further comprising a slippage determinationcanceling portion configured to inhibit determination of slippage whenthe continuously variable transmission is determined to be in theprocess of a shifting action based on a change in a running state of avehicle in which the continuously variable transmission is installed. 7.A method of detecting slippage in a continuously variable transmissioncapable of continuously changing a gear ratio between an input rotationspeed of an input member and an output rotation speed of an outputmember, comprising the steps of: calculating a sum of differencesbetween an actual gear ratio calculated from measurement values of theinput rotation speed and the output rotation speed and a target gearratio over a predetermined period of time; and determining slippage inthe continuously variable transmission based on the calculated sum ofdifferences.
 8. The method according to claim 7, wherein: thecontinuously variable transmission includes the input member, the outputmember, and a torque transmitting member for transmitting torque betweenthe input member and the output member; and the determining stepdetermines slippage of the torque transmitting member in thecontinuously variable transmission.
 9. The method according to claim 7,wherein slippage in the continuously variable transmission is determinedbased on the number of times the sum of differences exceeds apredetermined reference value within a predetermined period of time. 10.The method according to claim 7, wherein slippage in the continuouslyvariable transmission is determined based on a period of time for whichthe sum of differences is kept larger than a predetermined referencevalue.
 11. The method according to claim 7, wherein the target gearratio is determined based on a value obtained as a result of a smoothingoperation performed on a target input rotation speed that is determinedbased on an output requirement of a vehicle in which the continuouslyvariable transmission is installed.
 12. The method according to claim 7,further comprising the step of inhibiting determination of slippage whenthe continuously variable transmission is determined to be in theprocess of a shifting action based on a change in a running state of avehicle in which the continuously variable transmission is installed.